1. Field of the Invention
The present invention relates to a sheet finisher mounted on or operatively connected to a copier, printer or similar image forming apparatus for stapling, punching, jogging or otherwise processing sheets or recording media carrying images thereon and then cutting sheets, and an image forming system using the same.
2. Description of the Background Art
There is extensively used a sheet finisher positioned at the downstream side of an image forming apparatus for, e.g., stapling a stack of sheets sequentially driven out of the image forming apparatus. Today, even a sheet finisher with multiple advanced functions including an edge and a center stapling function is available. However, a sheet finisher with such multiple functions is, in many cases, bulky or is limited as to the individual function because of the combination of various functions. For example, Japanese Patent
Laid-Open Publication Nos. 07-48062 and 2000-153947 each propose a sheet finisher in which a path is switched at the inlet of the finisher to implement an edge and a center stapling function independent of each other. Although this kind of sheet finisher is feasible for a unit configuration and less-option application, combining similar functions is undesirable from the cost standpoint.
Further, in a center staple mode, the above sheet finisher is configured to jog and staple a sheet stack and then fold the sheet stack at the same position. This brings about a problem that the sheet finisher cannot deal with sheets belonging to the next job until it fully folds the sheets of the preceding job, resulting in low productivity.
In light of the above, Japanese Patent Laid-Open Publication Nos. 2000-118861 and 7-187479, for example, each disclose a sheet finisher of the type jogging and stapling, in an edge or a center staple mode, a sheet stack on a staple tray, which is inclined upward to the downstream side, switching back the stapled sheet stack to another tray positioned below the staple tray, and then folding the sheet stack. In this type of sheet finisher, a folding mechanism is independent of the other mechanisms and enhances productivity while minimizing an increase in cost ascribable to overlapping mechanisms. However, to enhance productivity, the staple tray is located at a high level in order to make the folding mechanism sufficiently long. As a result, two trays are connected together in a “<” configuration and make the entire sheet finisher bulky.
On the other hand, Japanese Patent Laid-Open Publication No. 2000-63031 teaches a sheet finisher configured to fold a sheet stack extending from a staple tray, thereby reducing the size of the sheet finisher. This, however, prevents productivity from being enhanced. Further, Japanese Patent Laid-Open Publication Nos. 11-286368 and 2000-86067 each propose a sheet finisher in which a fold roller pair is positioned slightly above the center portion of a staple tray so as to directly fold a stapled sheet stack, thereby implementing the shared use of a tray or reducing the length of a path. However, this configuration not only fails to enhance productivity, but also increases the size of the sheet finisher because the fold roller pair is positioned above the staple tray, which is inclined upward to the downstream side. In addition, a folded sheet stack is driven out of the sheet finisher at a relatively high level, so that the amount of usual edge-stapled sheet stacks that can be stacked is reduced.
Japanese Patent Laid-Open Publication Nos. 2000-198613 and 2000-103567 each disclose a value-added sheet finisher additionally provided with an edge cutting function. Such a sheet finisher includes either one of a guillotine type of cutter movable up and down and a shuttle type of cutter customary with, e.g., a facsimile apparatus or a plotter. Conventional sheet finishers each using the guillotine type of cutter or the shuttle type of cutter have the following problems (1) through (5) left unsolved.
(1) The cutter taught in the above Laid-Open Publication No. 2000-103567, for example, is a guillotine type of cutter. Generally, although a guillotine type of cutter is bulky and needs a large-output drive source, it has a sufficient height in a portion for delivering a sheet stack to a cutting portion and therefore does not need special means for insuring conveyance. However, in the case where a sheet stack is directly conveyed to a cutter portion by a roller pair just preceding the cutter portion, conveyance quality is questionable and will be a grave issue in consideration of further size reduction expected in the future.
The sheet finisher of Laid-Open Publication No. 2000-198613 also mentioned earlier includes an angularly movable guide plate just preceding a cutting portion and retractable in accordance with the movement of an elevatable cutting edge. However, this guide plate scheme is not easily applicable to the shuttle type of cutter, because the direction in which a shuttle moves and the direction in which the guide plate retracts would be perpendicular to each other. Further, while the guillotine type cutter allows sheet scraps to be easily dropped because of its movement, the shuttle type of cutter cannot do so and needs a sufficiently large opening for scraps to drop. Moreover, in the shuttle type of cutter, the opening is largest in the vicinity of the bottom dead center of a rotary edge, but slightly reduced at opposite sides of the bottom dead center. It is therefore likely that scraps staying around the rotary edge due to some cause close the opening when the rotary edge retracts.
(2) The shuttle type of cutter is feasible for a small size, power-saving configuration, as known in the art, and will probably be predominant over the guillotine type of cutter in the future. However, the probability of defective cutting increases with the shuttle type of cutter when it comes to small-size configuration. Further, if a sufficient cut margin is not available for structure reasons, then scraps are likely to curl and wrap around the rotary edge, causing an error to occur. When this kind of error occurs during cutting, the rotary edge stops while nipping a sheet stack and makes it impossible to remove the sheet stack. Generally, while the guillotine type of cutter allows such an error to be simply detected if one rotation of a cam is detected, the shuttle type of cutter cannot do so because it moves horizontally.
Other sheet finishers using the shuttle type of cutter are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-62262, 2001-88384 and 5-88271. Among them, the sheet finisher of Laid-Open Publication No. 2000-62262 is configured to reduce the cutting time when a medium has a small width, but does not addresses to an error to occur when a sheet stack is being cut. The sheet finisher of Laid-Open Publication No. 2001-88384 is configured to estimate the time for replacing a cutter and cause a replacement time sensing portion to output an alarm message or an alarm tone meant for the user. Further, the sheet finisher of Laid-Open Publication No. 5-88271 contemplates to promote easy replacement of a sheet stack jamming a path. For this purpose, this sheet finisher determines, based on whether or not a cutter has returned to its initial position within a preselected time, whether or not a jam has occurred. Even when a jam has occurred, the sheet finisher continuously drives the cutter to fully cut a sheet stack, prepares a magazine adjacent the cutter for removal, and then displays the jam.
(3) With the guillotine type of cutter, it is possible to make a cut margin noticeably small by adjusting alignment of both cutting edges. On the other hand, if the cut margin is extremely small, then the shuttle type of cutter causes scraps to deform like curled strips and causes them be caught by the rotary edge.
(4) Another problem with the shuttle type of cutter is that the rotary edge has a relatively small diameter, so that a load noticeably varies when the rotary edge starts cutting a relatively thick sheet stack. Consequently, a force tending to shift the sheet stack acts on the sheet stack and causes it to be shifted or scratched. Further, when use is made of a stepping motor, it is likely that the motor fails to follow the sharp change in load and is brought out of synchronism.
(5) The guillotine type of cutter cuts the entire sheet stack in a relatively short time, so that the resulting scraps drop to a position substantially beneath the sheet stack. Therefore, scraps cut away from consecutive sheet stacks are sequentially piled up around the center of the sheet stack because sheets are generally conveyed with the center as a reference without regard to the sheet size. Because a hopper for storing the scraps has a sufficiently larger width than the sheet width, the pile of scraps naturally collapses and can be stored in the hopper in a large amount.
On the other hand, the shuttle type of cutter cuts a sheet stack in one direction over a substantial period of time, so that the resulting scraps hang down from the sheet stack until the sheet stack has been fully cut. Consequently, the scraps fully cut away from the sheet stack drop to a position adjacent a position where the cutting stroke ends and shifted from the center of a hopper. One side of such scraps lean on the wall of the hopper. As a result, the pile of scraps does not naturally collapse and cannot be stored in the hopper in a large amount, as will be described more specifically later. Although the hopper may be provided with a larger capacity or a width sufficiently larger than that of a sheet stack, this kind of scheme increases the size of the entire sheet finisher and makes the use of the shuttle type of cutter practically meaningless.